Oxidizing Particulate Filter

ABSTRACT

A particulate filter assembly for application to an exhaust system of an internal combustion engine comprises an exhaust gas particulate filter configured to receive and filter exhaust gas from an exhaust system. An oxidation catalyst compound is disposed on an outer radial region of the exhaust gas particulate filter and is configured to induce oxidation of carbon monoxide from the combustion of carbon and particulates, to thereby maintain temperatures in the radially outer portion at a level sufficient to maintain the combustion of carbon and particulates therein.

FIELD OF THE INVENTION

Exemplary embodiments of the present invention relate to exhaust gas treatment systems for internal combustion engines and, more particularly, to an efficient system for assuring complete regeneration of an exhaust particulate filter.

BACKGROUND

The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NO_(x)”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.

An exhaust treatment technology, in use for high levels of particulate mater reduction, is the Diesel Particulate Filter device (“DPF”). There are several known filter structures used in DPF's that have displayed effectiveness in removing the particulate matter from the exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.

The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration of a DPF in vehicle applications is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF to levels that are often above 600° C. in order to burn the accumulated particulates.

One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the DPF. The HC may be delivered by injecting fuel directly into the exhaust gas system or may be achieved by “over-fueling” the engine. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the DPF and burns the particulate accumulation. Another method for generating temperatures sufficient to regenerate the DPF has involved the placement of an electric heater adjacent to the upstream face of the filter. When energized, the electric heater operates to deliver thermal energy to the upstream face of the filter that is sufficient for the ignition of the filtered particulates.

While these methods of heating the DPF are both effective for regenerating an un-catalyzed particulate trap, it has been found that heat loss from the outer surface of the DPF may result in incomplete combustion of the particulate matter in areas of the filter that are closely adjacent to its outer surface.

SUMMARY OF THE INVENTION

In an exemplary embodiment of the present invention, a particulate filter assembly for application to an exhaust system of an internal combustion engine comprises an exhaust gas particulate filter configured to receive and filter carbon and particulates from exhaust gas flowing through an exhaust system. An oxidation catalyst compound is disposed on an outer radial region of the exhaust gas particulate filter and is configured to induce oxidation of carbon monoxide from the combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein.

In another exemplary embodiment of the present invention, an exhaust gas particulate filter system for an internal combustion engine comprises, an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, an internal combustion engine, a hydrocarbon supply connected to and in fluid communication with the exhaust gas conduit for delivery of a hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture therein, an oxidation device downstream of the hydrocarbon supply, and configured to receive the exhaust gas and hydrocarbon mixture and induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas, and a particulate filter assembly in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein. The particulate filter assembly comprises an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas and an oxidation catalyst compound disposed on an outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates.

In yet another exemplary embodiment of the present invention, an exhaust gas treatment system for an internal combustion engine comprises an internal combustion engine and an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, the internal combustion engine and to conduct the exhaust gas between a plurality of devices of the exhaust gas treatment system. A selective catalyst reduction device, configured for reduction of components of NO_(x) in the exhaust gas, is disposed in fluid communication with the exhaust gas conduit. A hydrocarbon injector is connected to the exhaust gas conduit and is in fluid communication with the exhaust gas for delivery of hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture therein. An oxidation device is located downstream of the hydrocarbon injector, and is configured to receive the exhaust gas and hydrocarbon mixture and to induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas. A particulate filter assembly is in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and is configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein. The particulate filter assembly comprises an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas and an oxidation catalyst compound disposed on a outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein.

The above features and advantages, and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, advantages and details appear, by way of example only, in the following detailed description of the embodiments, the detailed description referring to the drawings in which:

FIG. 1 is a schematic view of an exhaust gas treatment system for an internal combustion engine;

FIG. 2 is a schematic, sectional view of an exemplary embodiment of a diesel particulate filter device embodying aspects of the present invention; and

FIG. 3 is a perspective view of the diesel particulate filter of FIG. 2 taken at section 3-3.

DESCRIPTION OF THE EMBODIMENTS

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

Referring now to FIG. 1, an exemplary embodiment of the invention is directed to an exhaust gas treatment system, referred to generally as 10, for the reduction of regulated exhaust gas constituents of an internal combustion engine, such as diesel engine 12. It is appreciated that the diesel engine 12 is merely exemplary in nature and that the invention described herein can be implemented in various engine systems implementing a particulate filter. Such engine systems may include, but are not limited to, gasoline direct injection systems and homogeneous charge compression ignition engine systems. For ease of description and discussion, the disclosure will be discussed in the context of a diesel engine.

The exhaust gas treatment system includes an exhaust gas conduit 14, which may comprise several segments that function to transport exhaust gas 16 from the diesel engine 12 to the various exhaust treatment devices of the exhaust gas treatment system 10. The exhaust treatment devices may include a first Diesel Oxidation Catalyst device (“DOC1”) 18. The DOC1 may include a flow-through metal or ceramic monolith substrate 20 that is wrapped in an intumescent mat (not shown) that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell or canister 21 having an inlet and an outlet in fluid communication with exhaust gas conduit 14. The substrate 20 has an oxidation catalyst compound (not shown) disposed thereon. The oxidation catalyst compound may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combinations thereof. The DOC1 18 is useful in treating unburned gaseous and non-volatile HC and CO, which are oxidized to form carbon dioxide and water.

A Selective Catalytic Reduction device (“SCR”) 22 may be disposed downstream of the DOC1 18. In a manner similar to the DOC1, the SCR 22 may also include a flow-through ceramic or metal monolith substrate 24 that is wrapped in an intumescent mat (not shown) that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell or canister 25 having an inlet and an outlet in fluid communication with exhaust gas conduit 14. The substrate 24 has an SCR catalyst composition (not shown) applied thereto. The SCR catalyst composition preferably contains a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) that can operate to effectively convert NO_(x) constituents in the exhaust gas 16 in the presence of a reductant such as ammonia (‘NH₃”). The NH₃ reductant 23, supplied from reductant supply tank 19 through conduit 17, may be injected into the exhaust gas conduit 14 at a location upstream of the SCR 22 using an injector 26, in fluid communication with conduit 17, or other suitable method of delivery of the reductant to the exhaust gas 16. The reductant may be in the form of a gas, a liquid or an aqueous urea solution and may be mixed with air in the injector 26 to aid in the dispersion of the injected spray. A mixer or turbulator 27 may also be disposed within the exhaust conduit 14 in close proximity to the injector 26 to further assist in thorough mixing of the reductant with the exhaust gas 16.

Referring to FIGS. 1, 2 and 3, an exhaust gas particulate filter assembly, in this exemplary case a Diesel Particulate Filter device (“DPF”) 28, is located within the exhaust gas treatment system 10, downstream of the DOC1 18 and the SCR 22, and operates to filter the exhaust gas 16 of carbon and other particulates. The DPF 28 may be constructed using an exhaust gas particulate filter such as ceramic wall flow monolith filter 30 that is wrapped in an intumescent mat 33 that expands when heated, securing and insulating the filter which is packaged in a stainless steel shell or canister 31 having an inlet and an outlet in fluid communication with exhaust gas conduit 14. The ceramic wall flow monolith has a plurality of longitudinally extending passages 32 that are defined by longitudinally extending walls 34. The passages 32, FIG. 2, include a subset of inlet passages 36 that have an open inlet end 38 and a closed outlet end 40, and a subset of outlet passages 42 that have a closed inlet end 44 and an open outlet end 46. Exhaust gas 16 entering the filter 30 through the inlet ends 38 of the inlet passages 36 is forced to migrate through adjacent longitudinally extending walls 34 to the outlet passages 42. It is through this wall flow mechanism that the exhaust gas 16 is filtered of carbon and other particulates 48. The filtered particulates are deposited on the longitudinally extending walls 34 of the inlet passages 36 and, over time, will have the effect of increasing the exhaust gas backpressure experienced by the diesel engine 12. It is appreciated that the ceramic wall flow monolith filter 30 is merely exemplary in nature and that the DPF may include other exhaust gas particulate filters such as wound or packed fiber filters, open cell foams, sintered metal fibers, etc.

In an exemplary embodiment, the increase in exhaust backpressure caused by the accumulation of particulate matter 48 requires that the DPF 28 is periodically cleaned, or regenerated. Regeneration involves the oxidation or burning of the accumulated carbon and other particulates 48 in what is typically a high temperature (>600° C.) environment. For regeneration purposes, an electrically heated catalyst device (“EHC”) 50 may be disposed within canister 31 of the DPF 28. The EHC 50 may be constructed of any suitable material that is electrically conductive such as a wound or stacked metal monolith 52. An electrical conduit 54 that is connected to an electrical system, such as a vehicle electrical system, supplies electricity to the EHC 50 to thereby heat the device, as will be further described below. In an exemplary embodiment, an oxidation catalyst compound (not shown) may be applied to the EHC 50 as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof.

In an exemplary embodiment, the ceramic wall flow monolith filter 30 has an oxidation catalyst compound 58 disposed about the perimeter thereof. The oxidation catalyst compound 58 extends, from the outermost surface of the wall flow monolith filter 30, radially inwardly a distance (“r”). The oxidation catalyst compound may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. Application of the oxidation catalyst compound 58 may be through a precision coating process in which a mold of the inlet end of the wall flow monolith filter 30 is made and through which the catalyst is machine dispensed or vacuum applied or both. Simpler coating techniques may include masking of the central portion 60 of the wall flow monolith filter 30 but such methods are more suitable to lower volume productions. The application of the oxidation catalyst compound 58 to the outer radial region 56 of the wall flow monolith filter 30 is for the purpose of assisting in the regeneration of the carbon and other particulates 48 that are trapped near the outer surface of the DPF 28 in regions that may be subjected to heat transfer rates from the canister 31 that may lower the regeneration temperatures to a level below which the particulates will not combust. The radial distance “r” or thickness of application of the oxidation catalyst compound 58 as well as the axial extent to which the oxidation catalyst compound 58 is applied to the wall flow monolith filter 30 is selected based on the heat transfer characteristics of a given configuration of DPF 28. In some cases, it may be necessary to apply the catalyst compound 58 along the entire length of the outer radial region 56 of the wall flow monolith filter 30 and in others; a partial or axially zoned application will be suitable for complete combustion of particulates 48.

Referring again to FIG. 1, disposed upstream of the DPF 28, in fluid communication with the exhaust gas 16 in the exhaust gas conduit 14, is an HC or fuel injector 62. The fuel injector 62, in fluid communication with HC 65 in fuel supply tank 63 through fuel conduit 61, is configured to introduce unburned HC 65 into the exhaust gas stream for delivery to the DPF 28. A mixer or turbulator 64 may also be disposed within the exhaust conduit 14, in close proximity to the HC injector 62, to further assist in thorough mixing of the HC with the exhaust gas 16.

A controller such as vehicle controller 66 is operably connected to, and monitors, the exhaust gas treatment system 10 through signal communication with one or more sensors. As used herein the term controller may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

In an exemplary embodiment, a backpressure sensor 68, located upstream of DPF 28, generates a signal indicative of the carbon and particulate loading in the ceramic wall flow monolith filter 30. Upon a determination that the exhaust system backpressure has reached a predetermined level indicative of the need to regenerate the DPF 28, the controller 66 activates EHC 50 and raises the temperature of the EHC to a level suitable for rapid HC oxidation. A temperature sensor 70, disposed within the shell 31 of the DPF 28 monitors the exhaust gas temperature downstream of the EHC 50. When the EHC 50 has reached the desired operational temperature, the controller 66 will activate the HC injector 62 to deliver fuel into the exhaust gas conduit 14 for mixing with the exhaust gas 16. The fuel/exhaust gas mixture enters the DPF 28 and flows through the heated EHC 50 that induces a rapid oxidation reaction and resultant exotherm to thereby raise the exhaust gas temperature to a level (>600° C.) suitable for regeneration of the carbon and particulate matter 48 in the ceramic wall flow monolith filter 30. Following its exit from the EHC 50, the heated exhaust gas 16 flows downstream through the ceramic wall flow monolith filter 30 where it effectively combusts the carbon and other particulates 48 trapped therein. During the regeneration event, carbon (“C”) is oxidized in the presence of oxygen (“O₂”) to generate carbon dioxide (“CO₂”) and carbon monoxide (“CO”):

C+O₂→CO₂+CO

In the outer radial region 56 of the wall flow monolith 30 in which the ring of oxidation catalyst compound 58 has been applied, the carbon monoxide (“CO”) generated from the oxidation of the carbon particulates 48 is oxidized by the oxidation catalyst compound 58, in an exothermic reaction to create carbon dioxide (“CO₂”):

CO+O₂→CO₂+Thermal Exotherm

As a result of the exothermic reaction in the outer radial region 56 of the wall flow monolith 30 the temperatures required to achieve complete combustion of the carbon and particulates 48 is maintained and the DPF 28 is fully regenerated across its entire cross-section.

In another exemplary embodiment, it is contemplated that, in some circumstances the fuel injector 62 may be dispensed with in favor of engine control of the HC levels in the exhaust gas 16. In such an instance the controller such as vehicle controller 66 is operably connected to, and monitors, the exhaust gas treatment system 10 through signal communication with a number of sensors such as backpressure sensor 68. The backpressure sensor generates a signal indicative of the carbon and particulate loading 48 in the ceramic wall flow monolith filter 30 and, upon a determination that the backpressure has reached a predetermined level indicative of the need to regenerate the DPF 28, the controller 66 activates EHC 50 and raises the temperature of the EHC to a level suitable for rapid HC oxidation (about 450° C.). Temperature sensor 70 monitors the exhaust gas temperature downstream of the EHC 50 and when the EHC 50 has reached the desired operational temperature, the controller 66 will adjust the engine timing and rate/frequency of fueling to deliver excess, unburned HC into the exhaust gas conduit 14 for mixing with the exhaust gas 16. The reactions described in the previous discussion, however, remain the same. In either case, the controller 66 may monitor the temperature of the exothermic oxidation reaction in the EHC 50 and the ceramic wall flow monolith filter 30 through temperature sensor 70 and adjust the HC delivery rate of injector 62 to maintain a predetermined temperature.

The use of an oxidation catalyst compound 58 disposed in limited region of the wall flow monolith filter 30, particularly the outer radial region 56 of the filter, provides for complete regeneration of the filter 30 while minimizing or thrifting the quantity of precious metals necessary for application to each DPF 28. In addition, the radially inner area 60 of the wall flow monolith filter 30 is protected from thermal extremes that would result in the case of application of the oxidation catalyst compound 58 to the entire filter 30 and could lead to sub-optimal durability or even failure.

While the invention has been described using an electrically heated catalyst device (“EHC”) 50 to initiate the oxidation of the exhaust gas and hydrocarbon mixture to heat the exhaust gas 16 to a temperature sufficient to regenerate the DPF 28, it is contemplated that other oxidation devices such as a passive upstream oxidation catalyst device, (ex. DOC1 18) may also be used for the same purpose without deviating from the scope or intent of the present invention.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application. 

1. A particulate filter assembly for application to an exhaust system of an internal combustion engine comprising: an exhaust gas particulate filter configured to receive and filter carbon and particulates from an exhaust gas flowing through an exhaust system and to combust the carbon and particulates therein; and an oxidation catalyst compound disposed on an outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from the combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein.
 2. The particulate filter assembly for application to an exhaust system of an internal combustion engine of claim 1, further comprising: an oxidation device upstream of the exhaust gas particulate filter and configured to receive an exhaust gas and hydrocarbon mixture and induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas to combust the carbon and particulates in the exhaust gas particulate filter.
 3. The particulate filter assembly for application to an exhaust system of an internal combustion engine of claim 2, wherein the oxidation device comprises an electrically heated device.
 4. The particulate filter assembly for application to an exhaust system of an internal combustion engine of claim 1, wherein the oxidation catalyst compound includes a platinum group metal such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalyst, or a combination thereof.
 5. An exhaust gas particulate filter system for an internal combustion engine comprising: an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, an internal combustion engine; a hydrocarbon supply connected to and in fluid communication with the exhaust gas conduit and the exhaust gas for delivery of a hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture therein; an oxidation device downstream of the hydrocarbon supply, and configured to receive the exhaust gas and hydrocarbon mixture and induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas; a particulate filter assembly in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein, the particulate filter assembly comprising: an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas; and an oxidation catalyst compound disposed on an outer radial portion of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from the combustion of carbon and particulates, to thereby maintain temperatures in the outer radial portion at a level sufficient to maintain the combustion of carbon and particulates.
 6. The exhaust gas particulate filter system for an internal combustion engine of claim 5, wherein the oxidation catalyst compound includes a platinum group metal such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalyst, or a combination thereof.
 7. The exhaust gas particulate filter system of claim 5, wherein the oxidation device comprises an electrically heated device disposed upstream of the exhaust gas particulate filter.
 8. The exhaust gas particulate filter system of claim 5, wherein the exhaust gas particulate filter further comprises: a ceramic monolith having exhaust flow passages extending therethrough defined by longitudinally extending porous walls therebetween, the exhaust flow passages comprising: a first subset of inlet passages having an open inlet end and a closed outlet end; and a second subset of outlet passages having a closed inlet end and an open outlet end, wherein the ceramic monolith is configured to receive the exhaust gas through the inlet passages and to migrate the exhaust gas through the longitudinally extending porous walls to the outlet passages and remove particulates from the exhaust gas.
 9. An exhaust gas treatment system for an internal combustion engine comprising: an internal combustion engine; an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, the internal combustion engine and to conduct the exhaust gas between a plurality of devices of the exhaust gas treatment system; a selective catalyst reduction device, configured for reduction of components of oxides of nitrogen in the exhaust gas, disposed in fluid communication with the exhaust gas conduit; a hydrocarbon injector connected to the exhaust gas conduit in fluid communication with the exhaust gas for delivery of hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture; an oxidation device downstream of the hydrocarbon injector, and configured to receive the exhaust gas and hydrocarbon mixture and induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas; a particulate filter assembly in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein, the particulate filter assembly comprising: an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas; and an oxidation catalyst compound disposed on an outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein.
 10. The exhaust gas particulate filter system for an internal combustion engine of claim 9, wherein the oxidation catalyst compound includes a platinum group metal such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalyst, or a combination thereof.
 11. The exhaust gas particulate filter system of claim 9, wherein the oxidation device comprises an electrically heated device disposed upstream of the exhaust gas filter.
 12. The exhaust gas particulate filter system of claim 9, wherein the exhaust gas particulate filter further comprises: a ceramic monolith having exhaust flow passages extending therethrough defined by longitudinally extending porous walls therebetween, the exhaust flow passages comprising: a first subset of inlet passages having an open inlet end and a closed outlet end; and a second subset of outlet passages having a closed inlet end and an open outlet end, wherein the ceramic monolith is configured to receive the exhaust gas through the inlet passages and to migrate the exhaust gas through the longitudinally extending porous walls to the outlet passages and remove particulates from the exhaust gas. 